Generally, the above type of speed change operating device is required to turn the operating lever to a proper position corresponding to a desired speed change stage of a derailleur. In this case, it is difficult for the speed-change operating device having a friction mechanism for applying to the lever a resistance overcoming a spring force of a return spring at the derailleur to operate the lever to move it to the proper position. Hence, the lever usually is over-shifted and a cyclist becomes aware by sensing with his leg during pedalling whether engagement of the driving chain with the desired sprocket has been accomplished and if not, he then returns the lever to an extent of the over shift.
Such operation, however, requires not only skill on the part of the cyclist, but also tends to result in nervous control of the lever. Also the time of operating the lever from a start of the speed change to a release of the lever after restoring the overshift is quite long. In other words, the time during which the cyclist is unable to devote his full attention on steering the bicycle, but rather on the speed change, will increase, thereby creating a time loss, which becomes especially critical in e.g., a road race.
To solve the above problem, a speed-change operating device has been proposed which is provided with a positioning mechanism comprising a positioning member having engaging portions corresponding to the respective speed change stages at the derailleur and an engaging member engageable with each engaging portion, so that the operating lever is operable always properly to the position corresponding to the desired speed change stage.
The operating lever, when the above positioning mechanism is assembled with the speed-change operating device, is operated only to switch the engaging member to one engaging portion to thereby ensure the engagement of the driving chain with the desired sprocket to reduce the operating time of the lever and to enable the cyclist to be free from the nerve-racking overshift operation and to concentrate his attention only on driving and steering the bicycle.
The positioning mechanism, however, is not usable without modification when the combination of the tooth number of the sprockets and the intervals therebetween change, because an operating wire connecting the speed-change operating device the derailleur should be adjusted in length to mate the position set by the positioning mechanism with the speed change stage at the derailleur and also a similar adjustment is required when the wire is elongated.
For the usual bicycle, the position adjustment need only be carried out, but for bicycles used for road racing, the bicycle wheel itself is required to be exchanged during the race. In a case where a multistage sprocket assembly attached to the wheel is changed when the wheel is exchanged, (in other words, the speed change stage of the derailleur does not mate with each sprocket) the position adjustment is required, which takes much time, resulting in a time delay in the race. As a result, the above type of speed change operating device having the positioning mechanism has been unusable in connection with racing bicycles.